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GPSYCHO - outer_loop() Algorithm

Based on Bosi et al. "ISO/IEC MPEG-2 AAC", J. Audio Eng. Soc. 45 (1997) p 789-814.

Another good complement to the ISO documentation is Brandenburg & Stoll, "ISO-MPEG-1 Audio: A Generic Standard for Coding of High-Quality Digital Audio", J. Audio Eng. Soc 42 (1994) p 780-792.

The goal of the outer_loop routine in MP3 is to find the combination of scalefactors within each scalefactor band which produce the least amount of audible distortion. Audible distortion is distortion in a scalefactor band which exceeds the masking thresholds (computed by the psycho-acoustic model)


initialize all scalefactors to 0. compute initial q = quantization step size (bin_search_stepsize) divide & conquer algorithm to find approximate value of q outer_loop: do { compute quantization with given scalefactors and not too many bits (call inner_loop) calc_noise(): compute distortion within each scalefactor band compare distortion to allowed distortion (from psy-model) over = number of scalefactor bands where distortion > allowed_distortion tot_noise = average, over all bands of distortion(db) - allowed_distortion(db) over_noise= as tot_noise, but only bands with distortion > allowed_distortion if this quantization is the best one found so far (determined by quant_compare()) save it. if over=0 we are done, exit. amplify scalefactors for bands with distortion } while over<>0 or !(all scalefactors set to their max) Restore BEST quantization

Whenever a scalefactor band is amplified, it will force the next quantization to use more bits for that band. This will result in more bits used to encode the MDCT coefficients in that band, and thus less quantization error. That is why bands with audible distortion are amplified. However, it will also result in less bits for the unamplified bands. But these bands had a quantization error less than the allowed masking, so hopefully they can tolerate a little more noise. The whole procedure is designed to allocate the bits to the bands which need them the most.

When the loop is done, if we found a quantization with count=0, everything is great. Otherwise, we have to choose the best quantization that we found. The ISO model chooses the last quantization tried during outer_loop. This is strange because this is usually one of the worst. The MPEG2 paper makes the obvious point that after trying out all the different combinations, you should choose the BEST one, not the LAST one!

GPSYCHO defines the BEST quantization via the subroutine quant_compare(), and this subroutine is in a constant state of flux!

If you have ideas for a better way to define the BEST quantization, let me know!

Gabriel Bouvigne makes the following point: Which do you think is worse?

The outer_loop() described above will choose the over=1 as the best quantization.